Tubular Metal Prosthesis and Method of Making It

ABSTRACT

For welding a polished component of one metal to a polished prosthesis of another metal, ramp surfaces are used, to compensate for loss of precise dimensions of the two components at the surfaces that face each other at the weld interface, during the respective polishing procedures for the two metals.

TECHNICAL FIELD

This invention relates to a method of making a machined and polishedtubular metal prosthesis that defines a lumen around a long axis of theprosthesis and to which is welded a component of another metal that hasopposed major surfaces that are arcuate luminal and abluminal surfacesrelative to said long axis.

BACKGROUND PRIOR ART

One disclosure of such a prosthesis is to be found in WO-A-02/15820 ofthe present applicant. The WO document discloses a self-expanding stentof nickel titanium shape memory alloy that carries a ring of spoons oftantalum at each end of the tube of the stent, these tantalum spoonsserving as radiopaque markers so that the stent prosthesis can belocated by radiographic techniques, when inside the body of a patient.

It is customary to polish a prosthesis during manufacture, prior toimplantation in the body, for the obvious reason that, at the moment ofimplantation, there should be no surfaces in a condition of anythingless than full integrity. Polishing is conventionally accomplished by anelectro-polishing procedure and, in electro-polishing, the rate ofremoval of solid material from the surface being polished will vary,according to the chemical composition of the surface and the chemicalcomposition of the fluid medium in contact with that surface. Forexample, in the case of a NITINOL nickel titanium memory metal stentwith tantalum radiopaque markers, electropolishing can remove NITINOLfour times as fast as it removes tantalum. Indeed, the present inventorshave recognised a problem with electro-polishing a prosthesis such asthe one disclosed in the WO document because, for any given electrolyte,the rate of electro-polishing of the tantalum spoon is liable to besubstantially different from the rate of polishing of the nickeltitanium prosthesis material.

Polishing the tantalum spoon separate from the nickel titanium stentmatrix would foe one way to manage the rate of removal of material fromthe surface being polished. However, electro-polishing is always liableto introduce some uncertainty as to the precise dimensions of thepolished workplace. Thus, when two components of a workpiece are to beapproximated precisely, and then welded together, any electro-polishingprior to welding is liable to detract from the precision of placement ofthe respective components each side of the welding interface. Any suchloss of precision can reduce the level of confidence in the integrity ofthe weld because, ideally, the gap between the components at the weldinginterface, to be filled by weld metal, should be precisely defined, andconstant.

How, then, is one to reconcile integrity of the welded joint withprecision polishing of the components of different metals? This is theproblem that the present inventors addressed, and the present inventionrepresents a solution to that problem.

In this specification, the word “polishing” covers any method ofremoving edges, surface roughness or imperfections. Electropolishing isone way. Chemical polishing, such as etching, is another. Mechanicalpolishing, such as tumbling or sand-blasting, is yet another.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a method ofmaking a prosthesis as stated above is characterised by the steps of 1)polishing at least one of the component and the prosthesis; 2) providingthe respective complementary edge portions as ramp surfaces which, priorto said welding, can approach each other as the component moves in onedirection along the long axis relative to the prosthesis, and can moveaway from each other, vice versa, when the component is moved in theopposite sense along the axis; 3) moving the component and theprosthesis relative to each other along the said axis, to approximatethe respective ramp surfaces ready for welding; and 4) welding togetherthe approximated ramp surfaces.

In another aspect of the present invention, a tubular metal prosthesis,such as is made by the method above, is characterised in that thecomplementary respective edge portions are ramp surfaces which (in theabsence of welding) can approach each other as the component moves inone direction along the long axis relative to the prosthesis, and moveaway from each other vice versa when the component is moved in theopposite sense along the axis; and the ramp surfaces of at lease one ofthe component and the prosthesis that receives the component exhibitpolished surfaces.

It will appreciated that the ramp surfaces compensate for anyvariability in the amount of material removed from the complementaryedge portions during polishing prior to welding, in that relative axialmovement of the ramp surfaces with respect to each other can compensatefor more or less removal of material from the ramp surfaces duringpolishing. It may be that the relative axial positions of theprosthesis, and the component welded to it after welding vary to someextent but, in accordance with the invention, this is to be preferredover the situation where these relative axial positions are maintainedconstant, but at the cost of having a welding gap that varies accordingto the amount of material previously removed by polishing.

The reader will appreciate that some sort of tool or jig or clamp willbe required, in order to support the prosthesis and component inapproximated positions where the desired welding gap is presented forreceipt of weld metal. The construction of any such tool, jig or clamp,is a matter for the skilled reader. Whereas optimal automated productionof a stream of stents might require construction of a special tool, amore labour-intensive or custom-welding procedure would present nodifficulties for a stent manufacturer.

It is routine and conventional these days to machine a metal prosthesisfrom a tube of raw material. One way is by etching procedures, forexample, when the tube is of stainless steel. However, laser cutting ofa stent matrix in the cylindrical wall of the tubular workpiece is bynow an intensively used and well-understood method for making a metalprosthesis such as a self-expanding stent out of a tubular raw materialsuch as nickel titanium shape memory alloy (NITINOL trade mark). For theskilled stent manufacturer, therefore, it is not a matter of difficultyto program the laser cutter to provide ramp surfaces in complementaryedge portions of a tubular metal prosthesis and a component to be weldedto that prosthesis.

The tantalum spoons of the above-mentioned WO document have edgeportions that complement corresponding edge portions on the nickeltitanium stent, with the portions each side of the welding interfacehaving a recognisable male/female relationship, and with the femaleportion being exhibited on the radiopaque tantalum marker spoons orother component to be welded to the prosthesis. Such a male/femalerelationship is visualised for the ramp surfaces of the presentinvention. Indeed, it is envisaged that the male portion would exhibitan arrowhead shape with a tip at one end and a shank at the other andthat the component with the female edge portions would exhibit at leastone re-entrant portion for engaging behind the arrowhead of the maleportion.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show how thesame may be carried into effect, reference will now be made, be way ofexample, to the accompanying drawings, in which

FIG. 1 is a perspective view of a single welding interface between partof a tubular metal prosthesis and a component of a different metal to bewelded to that prosthesis;

FIGS. 2 to 5 show, respectively, plan views of four variations on thedesign concept visible in FIG. 1;

FIG. 6 shows part of a ring of male portions that extends around thecircumference of a tubular metal prosthesis; and

FIG. 7 shows (at slightly higher magnification) part of a ring ofcorresponding components of another metal, with female portions destinedto be welded to the respective projecting male portions shown in FIG. 6.

DETAILED DESCRIPTION

Referring to FIG. 1, a stent 10 similar to the one shown inWO-A-02/15820 has, at each end, a ring of nodes 12 between adjacentstruts 14, 15 of the strut matrix of the stent. A shank 16 extendsaxially away from each node 12 and the stent matrix as such, as far asthe root 18 of an arrowhead shape 20 that lies between symmetrical andcomplementary ramp surfaces 22 and 24 that converge to a tip 26 of thearrowhead 20 to define a male portion that is approximated with a femaleportion of a tantalum marker spoon 30 that has an abluminal majorsurface 32 and (not visible in FIG. 1) an opposed luminal major surface34 which faces the long axis of the stent tube so that the thickness orthe tantalum spoon 30 lies within an annulus centered on the long axisof the stent.

The female portion chat receives the arrowhead 20 is provided by opposedsymmetrical complementary ramp surfaces 36 and 38 which converge to theroot or base 40 of what can be seen as a V-shaped recess to receive theramp surfaces 22 an 24 of the arrowhead 20. At the base of the groove 40there is a cylindrical throughbore 42 which is provided for reducing therisk of crack-initiation and propagation from the root of the V-shapedgroove that receives the arrowhead 20.

In manufacture, the stent matrix (typically made of nickel titaniumshape memory alloy) is electro-polished before it is approximated withthe tantalum spoon 30. Likewise, the tantalum spoon 30, likely stillpart of a laser cut tube of tantalum and in the company of a pluralityof other tantalum spoons that extends around the axis of the tubularworkpiece (see the description in WO-A-02/15820) is alsoelectro-polished and, after separate electro-polishing of the tantalumcomponents and the nickel titanium components, the respective arrowheadmale portions 20 can be brought into approximation with thecorresponding female ramp surfaces of the respective tantalum spoons 30,ready for welding.

FIG. 1 does reveal a small welding gap between ramp surfaces 22 and 36,and surfaces 24 and 38. This is the gap that is filled with weld metalin consequence of the welding step of assembly of the tantalum spoons 30to the stent prosthesis 10. If the amount of material removed from therespective ramp surfaces during respective electro-polishing isdifferent form what was predicted, this need have no disturbing effecton the desired welding gap, provided that prior to welding there is ajudicious relative axial movement of the male and female portions so asto bring about the desired optimal welding gap between the respectiveramp surfaces.

Turning now to the remaining drawing figures, FIG. 2 is provided withreference numbers corresponding to those of FIG. 1 and, we think, needsno further text description. FIG. 3 differs only slightly from FIG. 2,in that the female ramp surfaces do not run as far as the marginal edgeof the tantalum component. Rather, the ramp surfaces are set back fromthe edge 50, in that the ramp surface 36 continues as an unrampedaxially aligned edge 52 and, likewise, ramp surface 38 continues as anaxially aligned straight edge 54, parallel to edge 52. Recessing thearrowhead “inside” the tantalum spoon avoids any adverse effects thatmight flow from having the acute angle between the arrowhead rampsurfaces 22 and 24 and the base 56 of the arrowhead lying outside theenvelope of the tantalum spoon 30.

Turning to FIG. 4, there is shown, two further variations. First, theramp surfaces are arcuate rather than straight, so the welding gap isnot mathematically constant but nevertheless changes with relative axialmovement of the ramp surfaces and, second, the arrowhead is gripped byre-entrant portions 60 that flank the shank 16. This engagement can be,as a matter of design preference, either resilient or of form-fitconception, with the arrowhead being introduced from radially inside oroutside the female receiving portion in the component 30.

FIG. 5 reveals a further variation, and difference from FIG. 4, in thatthe arrowhead shape includes two terminal barbs 70 which can beresilient enough to locate the arrowhead with the cavity in the spoon30. From FIGS. 6 and 7 it will be appreciated that a ring of maleprojections 20 can be located around the circumference of theprosthesis, and offered up to a corresponding ring of recesses, eachwith root 40, in respective components of another metal such as spoons30. Just as in WO 02/15820 mentioned above, the ring of spoons canconstitute a complete ring of material around the lumen of theprosthesis, with only laser cut gaps in between adjacent spoons.

In the case of welding a plurality of components (such as radiopaquemarkers) to a prosthesis, the components can be found all within acommon workpiece, from which they are parted after welding, as describedabove. Alternatively, each component can be separate from the outset,then prepared individually, and then welded individually to theprosthesis.

It will be appreciated that the prosthesis needs to be polished to ahigh level and, generally speaking, the excellence and integrity of thepolishing of the prosthesis overall should, if anything, be higher thanthe level of polishing of a radiopaque marker on the prosthesis.

The ramp surfaces that characterise the present invention can compensatefor variations, prior to welding, in the weld surfaces of the prosthesisand component, respectively.

1. A tubular metal stent, comprising: a stent body formed from a firstmaterial including: a first stent ring including struts connected bynodes positioned at a first end of the stent body, a plurality of shanksextending axially away from nodes on the first stent ring along alongitudinal axis of the stent body, each of the plurality of shankshaving a head portion, each head portion including first and secondpolished ramp surfaces converging and meeting at a point axially awayfrom the stent body; and a plurality of radiopaque markers formed from asecond material different from the first material, each of the markersincluding an end recess having complementary polished third and fourthramp surfaces that abut the polished first and second ramp surfaces whenthe head portion is inserted into the end recess.
 2. The stent accordingto claim 1 wherein the end recess of each of the radiopaque markersincludes axially aligned edges parallel to the longitudinal axis of thestent body adjacent the polished third and fourth ramp surfaces.
 3. Thestent according to claim 2 further comprising a second stent ringincluding struts connected by nodes positioned at a second end of thestent body opposite the first end of the stent body.
 4. The stentaccording to claim 3 further comprising a plurality of shanks extendingaxially away from nodes on the second stent ring along a longitudinalaxis of the stent body, each of the plurality of shanks having a headportion.
 5. The stent according to claim 4 wherein each head portionincludes first and second polished ramp surfaces converging and meetingat a point axially away from the stent body.
 6. The stent according toclaim 1 wherein the head portion polished first and second ramp surfacesand end recess complementary polished third and fourth ramp surfaceshave an arcuate shape.
 7. The stent according to claim 6 furthercomprising a second stent ring including struts connected by nodespositioned at a second end of the stent body opposite the first end ofthe stent body.
 8. The stent according to claim 7 further comprising aplurality of shanks extending axially away from nodes on the secondstent ring along a longitudinal axis of the stent body, each of theplurality of shanks having a head portion.
 9. The stent according toclaim 8 wherein each head portion includes first and second polishedramp surfaces converging and meeting at a point axially away from thestent body.
 10. A tubular metal stent, comprising: a stent body formedfrom a first material including: a first stent ring including strutsconnected by nodes positioned at a first end of the stent body, aplurality of shanks extending axially away from nodes on the first stentring along a longitudinal axis of the stent body, each of the pluralityof shanks having a head portion, each head portion including first andsecond polished ramp surfaces converging and meeting at a point axiallyaway from the stent body; and a plurality of radiopaque markers formedfrom a second material different from the first material, each of themarkers including an end recess having complementary polished third andfourth ramp surfaces that abut the polished first and second rampsurfaces when the head portion is inserted into the end recess whereinthe head portion polished first and second ramp surfaces and end recesscomplementary polished third and fourth ramp surfaces have an arcuateshape
 11. The stent according to claim 10 further comprising a secondstent ring including struts connected by nodes positioned at a secondend of the stent body opposite the first end of the stent body.
 12. Thestent according to claim 11 further comprising a plurality of shanksextending axially away from nodes on the second stent ring along alongitudinal axis of the stent body, each of the plurality of shankshaving a head portion.
 13. The stent according to claim 12 wherein eachhead portion includes first and second polished ramp surfaces convergingand meeting at a point axially away from the stent body.
 14. The stentaccording to claim 13 wherein each of the radiopaque markers furthercomprises a cylindrical throughbore at an end of the recess.
 15. Atubular metal stent, comprising: a stent body formed from a firstmaterial, including: a first stent ring including struts connected bynodes positioned at a first end of the stent body, a plurality of shanksextending axially away from nodes on the first stent ring along alongitudinal axis of the stent body, an arrow-shaped head extending fromeach of the plurality of shanks, each arrow-shaped head portionincluding first and second polished ramp surfaces; and a plurality ofradiopaque markers formed from a second material different from thefirst material, each of the markers including an end recess havingcomplementary polished third and fourth ramp surfaces that abut thepolished first and second ramp surfaces when the head portion isinserted into the end recess.
 16. The stent according to claim 15further comprising a second stent ring including struts connected bynodes positioned at a second end of the stent body opposite the firstend of the stent body.
 17. The stent according to claim 16 furthercomprising a plurality of shanks extending axially away from nodes onthe second stent ring along a longitudinal axis of the stent body, eachof the plurality of shanks having a head portion.
 18. The stentaccording to claim 17 wherein each head portion includes first andsecond polished ramp surfaces converging and meeting at a point axiallyaway from the stent body.
 19. The stent according to claim 18 whereineach of the radiopaque markers further comprises a cylindricalthroughbore at an end of the recess.